Data analysis
The ecosystem multifunctionality
(EMF) indexes of alpine steppes with and without shrub encroachment were
calculated by five plant functions (Cover, Height, SR , AGB, and
BGB) and seven soil functions (SW, SOC, MBC, STN, MBN,
NO3−−N and
NH4+−N), which were also reclassified
into aboveground ecosystem multifunctionality (AEMF) index and
belowground ecosystem multifunctionality (BEMF) index. These indexes
were calculated as follows:
\begin{equation}
EMF=\frac{1}{N}\sum_{i=1}^{N}\text{Zij}\nonumber \\
\end{equation}where N represents the total number of functions evaluated andZij is the standardization (Z-score) of the i th ecosystem
function in the j th plot.
We further used the relative interaction intensity (RII) to estimate the
changes magnitude of the ecosystem functioning in semi-arid alpine
steppes induced by shrub encroaching. The RII index was calculated as:
RII=(VSE−V) / (VSE + V)
where VSE and V are the values of ecosystem individual
functions in shrub encroached and non-encroached alpine steppes,
respectively. The RII values >0 indicating a positive
impact shrub encroachment on the individual and multiple ecosystem
functioning.
The independent sample t -test was used to test the differences in
vegetation characteristics, soil variables and the EMF indexes between
alpine steppes with and without shrub encroachment. Two-way analysis of
variance (ANOVA) tested the impacts of shrub type and shrub
encroachment, and their interactions on the individual and multiple
ecosystem functioning. We further used the Pearson correlation to
explore the relationships among the individual ecosystem functions as
well as the ecosystem multifunctionality indexes.